We present a method that permits the retrospective assessment of frequency changes in species, based on the evaluation of specimens in biological collections. The method assumes that the increase and decrease in the frequency of a species is reflected in the number of collected specimens. A comparison of the specimen numbers from different time periods allows for an evaluation of the frequency changes of a particular species, provided that the specimen numbers are corrected for the general collecting activity of each time period. We used a reference data set consisting of 10 521 specimens of 85 species to reflect general collecting activity. For 42 species of bryophytes in Switzerland, we calculated the 'relative collecting activity', i.e. the number of specimens of an individual species as a percentage of the number of specimens from the reference data set. We examined changes in the relative collecting activity between the periods 1850-1939 and 1940-1999, using a permutation test. The calculated results were further assessed, taking all background information on each single species into account. In seven cases, the resulting assessments differed from the test results. According to the assessments, 16 species showed a decline and four had increased. The frequency of seven species was considered stable. For the remaining 15, mainly rare species, reliable assessments depend on further study of their former and actual frequencies. When species analysed were arranged into three classes of rare, medium and high frequency, the results showed that the rare and medium frequency species underwent significant decline, whereas the common species were stable. The fact that 12 species of medium or high frequency have most probably declined is of particular interest.